1. Field of the Invention
This invention relates to thermosetting resin compositions useful for mounting onto a circuit board semiconductor devices, such as chip size or chip scale packages (“CSPs”), ball grid arrays (“BGAs”), land grid arrays (“LGAs”) and the like, each of which having a semiconductor chip, such as large scale integration (“LSI”), on a carrier substrate. Similarly, the compositions are useful for mounting onto circuit board semiconductor chips themselves. Reaction products of the compositions of this invention are controllably reworkable when subjected to appropriate conditions. And significantly, unlike many commercial rapidly curing underfill sealants (“snap cure underfills”), the inventive compositions possess an exotherm under 300 J/g or demonstrate package stability at 55° C. for 7 days, and therefore do not require special packaging to be transported by air courier, or special approval from international transportation authorities, such as the U.S. Department of Transportation, to permit such air transport.
2. Brief Description of Related Technology
The popularity of small digital recorders, cellular (or, mobile) telephone sets, portable digital music recorder/players (such as iPod's) and devices combining such features has made size reduction of LSI devices desirable. As a result, CSPs, BGAs and LGAs have been used to reduce the size of packages substantially to that of bare chips. CSPs, BGAs and LGAs improve the characteristics of the electronic device while retaining many of their operating features, thus serving to protect bare semiconductor chips, such as LSIs, and facilitate testing thereof.
Ordinarily, the CSP/BGA/LGA assembly is connected to electrical conductors on a circuit board by use of a solder connection. However, when the resulting CSP/BGA/LGA/circuit board structure is exposed to thermal cycling, vibration, distortion or is dropped, the reliability of the solder-connection between the circuit board and the CSP/BGA/LGA often becomes suspect. After a CSP/BGA/LGA assembly is mounted on a circuit board, the space between the CSP/BGA/LGA assembly and the circuit board is often now filled with a sealing resin (commonly referred to as underfill sealing) in order to relieve stresses caused by thermal cycling, thereby improving heat shock properties and enhancing the reliability of the assembled structure.
However, since thermosetting resin compositions that form cross linked networks when cured are typically used as the underfill sealing material, in the event of a failure after the CSP/BGA/LGA assembly is mounted on the circuit board, it is difficult to replace the CSP/BGA/LGA assembly without destroying or scrapping the CSP/BGA/LGA assembly-circuit board structure in its entirety.
U.S. Pat. No. 6,316,528 (Iida) refers to a thermosetting resin composition capable of sealing underfilling between a semiconductor device including a semiconductor chip mounted on a carrier substrate and a circuit board to which said semiconductor device is electrically connected. The composition includes about 100 parts by weight of an epoxy resin, about 3 to about 60 parts by weight of a curing agent, and about 1 to about 90 parts by weight of a plasticizer. There, the area around the cured thermoset is heated at a temperature of about 190 to about 260° C. for a period of time ranging from about 10 seconds to about 1 minute in order to achieve softening and a loss of much of its adhesiveness.
In addition, certain underfill sealants have been designed to cure rapidly at low temperatures. One of the technical challenges of designing such products is balancing the desired low temperature and rapid cure with shelf stability and low exotherm. Shelf stability is improved by cold storage and transport; however, the exothermic nature of the product is inherent to the product formulation.
International transportation regulatory authorities require reactive formulations to have either an exotherm below a certain value (300 J/g), demonstrate stability over a set period of time at slightly elevated temperatures or be packaged in specially designed packaging to be transported by air, and in some jurisdictions both. As an example of a specially designed packaging, see U.S. Pat. No. 6,070,427.
Clearly, it would be desirable to provide reactive formulations whose exotherms fall below that set value and as a result do not require the specially designed packaging, while providing the physical property profile desired by the end user.
Henkel Corporation introduced a snap cure underfill under Product No. 3593, which is a rapid curing, fast flowing, liquid designed for use as a capillary flow underfill, particularly for CSPs. Product No. 3593 has been well received commercially. However, certain physical properties of Product No. 3593, such as crack resistance and moisture uptake, could stand improvement.
Thus, it would be desirable to provide a thermosetting resin composition useful as an underfill sealant that possesses an exotherm under 300 J/g or demonstrates package stability at 55° C. for 7 days.
Notwithstanding the state-of-the-art, it would be desirable for an underfilling sealing material to be able to be transporting without requiring specially designed packaging in order to comply with international transportation authorities; to flow rapidly by capillary action in the underfill space between for instance the CSP, BGA or LGA and the circuit board; to cure rapidly under low temperature conditions; to provide good productivity and thermal shock resistance, while allowing the substrates with which it is to be used to be readily processed and easily separated from a semiconductor device without too extreme conditions that may compromise the integrity of the semiconductor devices remaining on the substrate or the substrate itself; and to be reworkable in the event of failure of the semiconductor in the CSP, BGA or LGA once assembled onto the circuit board.